The invention relates to the field of intravascular catheters, and more particularly to a balloon catheter or other catheter component, such as a guidewire enclosure, that would benefit from the properties of the materials disclosed herein.
In percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of a dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient's coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient's coronary anatomy, over the previously introduced guidewire, until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g. greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not over expand the artery wall. The rate of expansion of the balloon for a given pressure is an important consideration in the design of the dilation catheter, as greater than anticipated expansion of the balloon against the vessel wall can cause trauma to the vessel wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed from the patient's artery.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion.
In the design of catheter balloons, balloon characteristics such as strength, flexibility and compliance must be tailored to provide optimal performance for a particular application. Angioplasty balloons preferably have high strength for inflation at relatively high pressure, and high flexibility and softness for improved ability to track the tortuous anatomy. The balloon compliance is chosen so that the balloon will have a desired amount of expansion during inflation. Compliant balloons, for example balloons made from materials such as polyethylene, exhibit substantial stretching upon the application of tensile force. Noncompliant balloons, for example balloons made from materials such as PET, exhibit relatively little stretching during inflation, and therefore provide controlled radial growth in response to an increase in inflation pressure within the working pressure range. However, noncompliant balloons generally have relatively low flexibility and softness, making it more difficult to maneuver through various body lumens. Heretofore the art has lacked an optimum combination of strength, flexibility, and compliance, and particularly a low to non-compliant balloon with high flexibility and softness for enhanced trackability.
Another issue of concern in the use of balloons such as catheter balloons described above is the phenomena of shredding. Balloons are designed so that if a failure occurs, the failure is along the longitudinal axis rather than the circumferentially. This is because a failure balloon with a longitudinal failure can still be withdrawn from the patient's vascular, whereas a circumferential failure could result in the distal portion of the balloon becoming detached from the proximal portion, making retrieval problematic. Therefore, any rupture of the balloon is designed to propagate along the length of the balloon. To accomplish this disposition to tear axially, the balloon is imparted with a longitudinal elongation or orientation of the polymer chains that form the balloon's material. Because the stretching of the tubing in the axial direction is predominant over the radial orientation, the molecular chains of the polymer become stretched or elongated in the axial direction. This orientation creates a disposition for tears or shredding in the balloon to propagate along the longitudinal direction.
This predisposition to fail in the axial direction sometimes leads to premature failure, especially during the handling or cleaning process. When the balloon fails, typically strands of balloon material lift and separate from the body of the balloon, and these strands can break away or cause defects in the balloon that can fail during pressurization. Also, the balloon can fail as it is being inflated. Shredding is not limited to elongate strands and may include flakes, chunks, pits, and other discontinuities in the balloon. These defects raise two concerns: (1) that the separated portions of the balloon may become dislodged in use, entering the patient's blood stream and causing blockage, infection, or other issues; and (2) the balloon may rupture prematurely along the defects due to the loss of structural integrity. Therefore, a solution to the problem of balloon shredding is needed. One can reduce the propensity for shredding by decreasing the axial orientation of the blow molded balloon by thermally relaxing the outer layer while maintaining high axial and radial orientation of the inner layer, thus maintaining desired balloon properties. To reduce the axial orientation of the outer layer only, the outer layer is selected to have a relaxation temperature (glass transition temperature or melting temperature) that is lower than the relaxation temperature of the inner layer.
Another issue arises in the formation of the balloon, i.e. the “blowing” process. In a single layer balloon of a semi-crystalline polymer such as Pebax 72D, the modulus of elasticity drops off directly with an increase in temperature. At temperatures where the blowing process occurs, the modulus of elasticity is such that premature rupture frequently occurs, resulting in loss of product. To avoid premature rupture during the blowing process, a multi-layer or blended balloon is formed having a first material of one polyamide having a Shore D durometer value of greater than 77 such as amorphous polymers (e.g., Grilamid TR55) and a second semi-crystalline material of a lower Shore D durometer value, preferably less than 73D, such as Pebax 72D, Pebax 70D, Pebax 63D or Pebax 55D. The co-extrusion or blended material facilitate the blowing process by making the formation more controllable and the blowing procedure can occur at higher temperatures without premature rupture. The inclusion of the amorphous component provides enhanced modulus to the overall balloon, which allows expansion under higher pressures and temperatures without premature rupture.
Another issue arises when the balloon catheter is used as a stent delivery device. Stent needs to be securely mounted on the folded balloon so as not to dislodge during delivery of the stent to the target lesion. Typically a stent is crimped onto a folded balloon at an elevated temperature. In the case of a single layer balloon catheter having high melting temperature such as polyamide or copolyamide, to make the balloon soft enough to imbed the struts of the stent, one is limited on temperature at which the balloon can be exposed due to potential for causing mechanical damage by the stent struts. Having a balloon comprised of an amorphous outer layer such as polyimide having a Shore D durometer value of greater than 77 (e.g., Grilamid TR55) and inner layer a second semi-crystalline material of a lower Shore D durometer value, preferably less than 73D, such as Pebax 72D, Pebax 70D, Pebax 63D or Pebax 55D, allows stent crimping at lower temperatures due to lower glass transition temperature of amorphous outer layer when compared with the melting temperature of semi-crystalline inner layer.